We intend to elucidate mechanisms and dynamics of effective immunity versus specific immunosuppression in tumor growth using several syngeneic fibrosarcomas of methylcholanthrene origin in inbred rats. Effects of defined antibody fractions and of immunized lymphocytes will be evaluated quantitatively in several assays in vivo and in vitro. We shall further investigate specific versus cross-reactive components of both cell-mediated immunity and antibody responses. This will also entail characterization of purified antibodies and antigen-antibody complexes for cytotoxic versus immunoblocking effects in tumor recipients.